1. Field of the Invention
The present invention relates to a transceiver for a station of a network of the type in which a plurality of stations are connected via links to a network common point through which a transmission received from any of the stations passes on its way to all the stations, and in particular, but not exclusively, the present invention relates to optical transceivers for use in optical star networks incorporating passive star couplers.
2. Description of the Prior Art
In optical star networks with passive star couplers, the network stations are often arranged to operate autonomously, that is, without any common control. In this event, some form of collision detection is generally required to enable the stations to recognize a clash between two or more stations that start transmitting concurrently. An appropriate network protocol for such a situation would be a CSMA/CD (Carrier-Sense, Multiple Access/Collision Detection) protocol such as Ethernet. Collision detection is generally the responsibility of each station undertaking transmission, and upon detection of a collision, the detecting station is arranged to output a jam tone over the network to warn all other stations.
In coaxial-cable CSMA/CD LANS, collision detection is simple to implement. The transmissions from each station contain a d.c. voltage component. During transmission, each station monitors the line, watching for changes in the d.c. voltage level. If such a change occurs, then a collision can be assumed to have taken place. This technique works well because the d.c. attenuation of the medium is small over the length of a single link and the transmission of each station is designed to give a uniform output.
Collision detection in an optical star network with a passive star coupler is, however, considerably more difficult because variations in link attenuation and transmitter output make collision difficult to detect. For example, consider an optical network with link lengths of 0-500 m, a fibre attenuation of 3 dB/km, and LED transmitters with an average power output of -10 dBm+2 dB. The star coupler of the network is assumed to have an attenuation of 20 dB+2 dB. In such a network, a station close to the star coupler may receive -26 dBm of optical power from its own transmitter, but only -35.5 dBm of optical power from the transmitter of a more distant station. To detect a collision between itself and the more distant station, the station must be able to recognize that there are two simultaneous optical signals 10 dB apart, which is not easily achieved.
European Patent Application No. 0,216,214 (Siemens) describes an optical network with a passive star coupler in which each station has an optical transceiver that is operative both in a normal mode, in which it serves for the transmission and reception of data over the network, and in a calibration mode. The transceiver includes an optical transmitter the output level of which is adjustable, an optical receiver, and adjustment means connected to the transmitter and operative when the transceiver is in its calibration mode to set the output power level of the transmitter such that the resultant optical power produced at the star coupler of the network is at the same level for all stations. By setting the transmitter output power level to achieve this result, the received power level at any particular station will be the same regardless of which station is transmitting. Thus, should two stations transmit simultaneously, the power received at any station will be double, which, of course, is easily detectable.
Although the approach adopted in the foregoing European Application overcomes the problem of needing to detect signals of vastly different strengths when effecting collision detection, the calibration process effected by the adjustment means of the station transceiver is somewhat complex. More particularly, the transceivers must all be set into their calibration mode together, and thereafter the adjustment means of the transceivers carry out the following two calibration phases:
1. Each transmitter in turn sends out a calibration signal at maximum power and the received power level at each station is monitored; the adjustment means of each transceiver then operates to store the lowest received power level it has monitored during this first phase. PA0 2. Next, each transceiver in turn activates its transmitter and then reduces its output power level until the power level at its receiver equals the value of the lowest received power level stored in the first phase.
The overall effect is that the power level produced at the star coupler by each transmitter will be the same.
It will be apparent to those skilled in the art that the foregoing calibration process suffers from a number of disadvantages, the most notable being the need for the participation of all transceivers (so that each time a new station joins the network, the calibration process must be repeated) and the need for the transceivers to operate in turn and to listen to all other transceivers (which either requires each transceiver to have a table of all other stations or for inefficient assumptions to be made regarding the number of stations present always being the maximum).